The present invention relates to a motor control system, or more in particular to a motor control system low in cost and suitable for speed regulation with simple configuration.
A motor such as a sync motor does not generate any rotational torque at other than a sync speed and has a starting torque substantially equal to zero. As a result, a self-starting method with a starting (brake) winding or a starting auxiliary motor is required.
The sync motor is a constant-speed motor adapted for rotating at a sync speed and is difficult in speed control. By changing the sync speed, however, the speed control is possible. Specifically, the speed control of this type of motor can be attained by changing either the frequency of a power supply or the number of poles thereof. The method with a changing power frequency is applicable to the motor operated with an exclusive power generator. The method with the number of poles changed, on the other hand, requires the number of poles of the armature windings and the number of fields to be switched at the same time.
The above-mentioned conventional sync motor, however, is difficult to start, and requires a special starter, thereby making the configuration thereof complicated and the cost thereof high. In a sync motor, the load current of which is comparatively small but increases with the decrease in speed, it is difficult to regulate the speed thereof over a wide range from high to low levels. Further, if the sync motor is to be decreased in size and improved in efficiency, a strong permanent magnet is required as a rotor thereof. For this purpose, a very expensive rare earth magnet is used.
In order to resolve these problems, the inventors of the present invention have suggested "A Synchronous Motor Control System" as disclosed in JP-A-62-114494. In this sync motor control system, a conduction phase operation circuit compares an output of a three-phase rectangular wave oscillation circuit controlled by a frequency control signal with a phase control signal thereby to control the conduction phase and to control the start of a sync motor automatically. The conduction phase operation circuit used in such a conventional control system cuts off the current input unrequired for torque by differentiating a current value. With the position of the rotor magnet changed forward or backward with load, however, accurate control has actually been sometimes impossible.
In order to overcome this drawback, it was proposed by the same inventors as the present invention in Japanese Patent Application No. 62-259399 that at least a Hall sensor is mounted on the primary side of a power transformer (PT) in a stator box for detecting the position of the rotor and the conduction phase operation circuit compares a position detection signal from the Hall sensor with the phase control signal.
According to this control apparatus, the conduction phase operation circuit compares a position detection signal from the Hall sensor with the phase control signal thereby to effect accurate current control which would be insufficient by a mere differentiation of a current value.
As a result, more accurate automatic start control and a wider range of speed control of a sync motor become possible.
A sharp start-up of this control system may be impossible, however, in view of the fact that the motor is started by a control signal from a frequency regulation circuit for producing a frequency control signal giving an instruction to increase the oscillation frequency from zero linearly to a predetermined high frequency.
Further, although this control system would not pose any problem under such a load as may be allowed corresponding to the torque characteristic of the motor, the motor may step out and become impossible to start in the case where the load is displaced from a torque curve, that is, under a load beyond the control of the control system.
In order to overcome the above-mentioned inconvenience, there was proposed by the same inventors as the present invention in Japanese Patent Application No. 63-131012 a sync motor control system which comprises a multi-phase motor, a multi-phase rectangular wave oscillation circuit, a position detecting unit for detecting the position of magnetic poles of the rotor of the multi-phase sync motor, a first DC voltage generation circuit for generating a DC voltage proportional to the rotational frequency of the rotor, a second DC voltage generation circuit for generating a DC voltage corresponding to the frequency from the multi-phase rectangular wave oscillation circuit, a comparator circuit for comparing output signals of the aforementioned two DC voltage generation circuits with each other and producing an output signal only when the DC voltage of the latter exceeds that of the former, and a drive control circuit for applying a signal to the excitation drive circuit of the motor for driving the motor as a brushless motor in accordance with an output signal from the magnetic pole position detection unit upon application thereto of an output signal from the comparator circuit and applying a signal to the excitation drive circuit for driving the motor as a sync motor upon application thereto of an output signal from the multi-phase rectangular wave oscillation circuit.
In this apparatus, when a power switch is turned on, an oscillation circuit is energized to generate a predetermined frequency, which is taken out as a corresponding DC voltage at a second DC voltage generation circuit and is applied to a comparator. The voltage generated at a rotor magnetic pole position detection unit, on the other hand, is taken out as a DC voltage proportional to the rotational frequency of the rotor at a first DC voltage generation circuit and is also applied to the comparator. The comparator, comparing the two input voltages thereto, produces an output signal only when the voltage on the rotor side is lower than the voltage on the oscillation circuit side, that is, when the rotor speed fails to reach the sync speed. This output signal is applied to a drive control circuit thereby to start the motor as a brushless motor. When the voltage on the rotor side reaches the same level as that on the oscillation circuit side, that is, when the rotational speed of the rotor reaches the sync speed, the comparator does not produce any output signal, but the motor is driven as a sync motor on the basis of a sync motor drive signal sent from the oscillation circuit to the drive control circuit. As a result, at the time of starting, the motor is driven according to the starting characteristic of a brushless motor, and when a sync speed is reached, is switched to a sync motor, thus making it possible to start up the motor sharply and drive it in stable manner.
In the case where power is switched on in the control system mentioned above, however, the frequency of the three-phase rectangular wave oscillation reaches a designated value only after the lapse of a certain length of time (one second divided by several minutes), and therefore a false operation may develop in switching from brushless motor to synchronous mode. Further, the second and first DC voltages obtained by differentiation and integration from the three-phase oscillation frequency and the rotational frequency respectively as used for switching may develop a ripple in the value thereof or may be delayed in time, sometimes causing an unstable switching operation.